The present invention relates to a reinforced multilayer thermoplastic resin composite with at least one layer of radio frequency sensitive material and to a method of producing a composite by using radio frequency wherein said composite comprises at least one fiber-free surface.
A glass fiber reinforced composite product must demonstrate appropriate mechanical properties such as tensile, flexural and impact strength and possess a smooth, defect free surface on at least one side to replace parts formed from sheet metal panels. The suitability of using reinforced thermoplastic resin composites for vehicle body parts is currently being investigated. The sheet metal currently used for wide, thin body parts, such as the hood or trunk of an automobile, is a likely candidate for replacement by a glass fiber reinforced composite material.
Generally, such a glass fiber reinforced composite body part would be manufactured by providing layers of thermoplastic resin and fiber mat in the desired quantity and structural arrangement to form a laminated structure; by heating the laminate to a temperature in the range of about 200.degree. C. to about 375.degree. C. and by applying a pressure to the laminate in the range of about 5 lb/in.sup.2 to about 50 lb/in.sup.2, thereby forming a composite material blank and by shaping the composite blank by flow forming or compression molding processes to form the desired vehicle body part. While a composite material manufactured by such a process possesses the mechanical and flexural strength required of a vehicle body part, the high quality, smooth defect-free surface finish that is also required to meet automotive requirements for exterior body applications has been difficult to produce by the use of existing composite structures and processing techniques. A general discussion of existing processes for producing and for compression molding composites may be found in "Composites", Chou, T., et al., Scientific American, Vol. 255, No. 4, October 1986, pp. 192-203, and Krone, J. R., and Walker, J. H., "Processing Thermoplastic Advanced Composites", Plastics Technology, Vol. 32, No. 11, November 1986, pp. 61-5.
In a typical flow forming process the composite blank is heated in a conventional oven by convection or infrared radiation to a temperature in the range of about 200.degree. C. to about 375.degree. C. During the initial heating in the oven the fibers expand, resulting in a resin poor coating of their surface. In addition, this expansion of the fibers results in a lofting, or movement, of the fibers into the resin surface layers.
Following the oven heating, the composite is transferred to the mold where it is shaped by applying pressure in the range of about 1000 lb/in.sup.2 to about 4000 lb/in.sup.2 with mold surfaces whose temperatures range from about 65.degree. C. to 150.degree. C. During the transfer of the composite from the oven to the mold the composite surface cools and the surface resins "freeze" into position. This "freezing" of the resin at the surface prevents the resin from flowing readily during the molding process and, consequently, rough boundaries are produced between the newly formed surface areas and the original surface areas. .In addition, the resulting composite surface is only partially filled with resins, even though some hot resin will move from the composite core to the surface during the molding process. This partially filled resin surface, particularly around and near the lofted fibers, is a major cause of surface roughness.
An additional problem inherent in existing molding processes arises from conventional methods of heating the composite. Conventional heating during the compression molding operation relies upon heat conduction from the outer surface layers to melt the inner bulk layer. Thus, a temperature gradient is established across the composite from the surface layer to the interior. Prolonged heating to ensure that the inner bulk layer has melted can result in an oversoftening of the surface layer resin, which in turn facilitates the lofting of fibers into the surface layer, which, as already noted, can be a cause of surface roughness. This prolonged heating can also cause thermal decomposition of the resin. A final disadvantage of current production techniques, is the length of time required to heat the composite. Conventional heating by convection or infrared radiation, sufficient to melt the composite so that it can be molded, may take from eight to ten minutes due to the relatively poor thermal conductivity of thermoplastic resins. Valuable production time is thereby lost by use of a production method which, moreover, results in a product with a rough surface.
Certain processes for manufacturing thermoplastic resin products--as opposed to reinforced thermoplastic resin composites--utilize a heating technique known as radio frequency (RF) heating. RF heating utilizes the dielectric properties of a material to generate heat therein. When electromagnetic radiation at a selected radio frequency is applied to the material, the alternating electric field of the RF electromagnetic radiation causes an oscillatory displacement of the charged components of the material, thereby resulting in a rise in the temperature of the material. The amount of heat generated therein is given by the formula: EQU P=K.epsilon. tan.delta.
where
P=heat generated; PA1 K=constant dependent upon the frequency of the applied radiation, the electric field strength, the material dimensions, and the units used; PA1 .epsilon.=dielectric constant of the material; and PA1 tan.delta.=loss tangent or dissipation factor of the material.
The ability of a material to generate heat when exposed to RF radiation is referred to as its RF sensitivity.
RF heating has been used on thermoplastic resins that have traditionally been difficult to process, such as ultrahigh molecular weight polyethylene. It has been possible to mold and extrude such a thermoplastic resin with conventional equipment by the addition to it of RF sensitive material, i.e., material having a sensitivity greater than the material to which it is added.
It is an object of the present invention to provide a method of making fiber reinforced composite parts with smooth surfaces by flow forming processes.
It is a further object of the present invention to provide a composite blank suitable for making fiber reinforced composite parts by flow forming processes.
It is yet a further object of the present invention to provide a method of making fiber reinforced composite parts with smooth surfaces by flow forming processes that has fast melting of the resin in the composite and minimized decomposition of the resin at the surface.
It is another object of the present invention to provide a composite blank suitable for making fiber reinforced composite parts by flow forming processes that has fast melting of the resin in the composite and minimized decomposition of the resin at the surface.